I'm not saying all cash-only bars are laundering money, but I am saying that the owner is probably at the very least not declaring all of their profits.
It's usually just not declaring profits in my experience, it's brutally addictive to slap 30% ish profit on top of everything.
My boss at a pizza place had 2 registers he would use for various reasons but it all boiled down to one being reported to the IRS and the other not.
I know a liqour store that has a small bar inside thats cash only, I got friendly enough with a manger that basically admitted the locals that hung out there were mostly blue collar and came in after work where they got payed cash. Meanwhile the retail liqour customers largely were card users. So the owner made the bar cash so he could report less. Makes sense why its the cheapest bar around by a large margin.